HMRC's customer service is "poor value for money" , according to an NAO report that estimated delays in answering calls were costing taxpayers around £136m a year.
The NAO report on HMRC customer service acknowledged the tax department's attempts to restore service quality from the lows of 2010, but said it still had a long way to go to be on par with other governmental organisations.
As people have "no choice" in dealing with the Revenue, it's HMRC's duty to provide the public good customer service, the NAO said.
Some 20m calls went unanswered by HMRC in 2011-12, with an average of 74% of all calls answered - described by the NAO as a "low level of service".
However, the number of answered calls could be overestimated, as calls are deemed answered even when put through to an automated message.
HMRC spends roughly a quarter of its expenditure, £900m, on customer service each year.
"Despite some welcome improvements, HMRC has acknowledged that its performance in providing services to the public has been unacceptable." said head of the NAO Amyas Morse.